Just use the ref sheets

I'm asking how much I should charge not what I can show of them

You have to be dispassionate about it.
Sure you're selling the backstory, but functionally that's no different to selling a car.
Once the car leaves your hands, what happens to it is of no concern to you... otherwise you wouldn't be selling the character.

So you have to balance out the value of said characters, but acknowledge that as valuable as they are to you, there has to be some tangible market value that people are willing to pay for, mostly the backstory.

Your character *might* be worth a gorillion dollars to you, but although someone might be willing to pay for that (MilesDF, Strype...) , you'll be waiting a while.

But if you do expect a reasonable amount, you have to throw in a sweetener. Not just backstory, but *all* commissioned art of the character. As a gesture of giving up ownership, you'd probably want to delete all art off your account(s) of that character, too.

So unless you've got the prices of each and every commission down somewhere, come up with a reasonable price based on that. Probably have to come down a bit from your initial value.
How much it boils down to, comes down to how much you feel the character and their backstory is versus how much you think the market is willing to pay for it.